a) Field of the Invention
The present invention relates to an objective lens driving apparatus which is assembled into an optical head apparatus for regeneration of optical recording media and the like, in particular, it relates to an objective lens driving apparatus which can be structured thinner and a method of manufacturing it.
b) Description of the Related Art
U.S. Pat. No. 4,998,802 is an example of an objective lens driving apparatus which is assembled into an optical head apparatus wherein the apparatus comprises a lens holder to support an objective lens and a magnetic driving circuit to move the lens holder in the focusing direction and the tracking direction.
The structure of this objective lens driving apparatus is called a contactingly movable axis type in which the lens holder is moved along a support shaft in the focusing direction and the lens holder is rotated around the support shaft in the tracking direction.
A magnetic driving circuit to move the lens holder described above comprises a magnetic driving circuit for focusing to contactingly move the lens holder along the support shaft in the focusing direction and a magnetic driving circuit for tracking to rotate the lens holder around the support shaft in the tracking direction.
The magnetic driving circuit for focusing is structured such that a pair of driving coils for focusing is mounted on the outer surface of the lens holder to be symmetric around the support shaft and a pair of magnets for focusing is mounted on the inner surface of an outer yoke, which is formed to surround the outer surface of the lens holder, such that the magnets lay opposite from the driving coils.
In a similar manner, the magnetic driving circuit for tracking is structured such that a pair of driving coils for tracking is mounted on the outer surface of the lens holder to be symmetric around the support shaft and a pair of magnets for tracking is mounted on the inner surface of an outer yoke, which is formed to surround the outer surface of the lens holder, such that the magnets lay opposite from the driving coils.
Flat hollow coils adhered to the outer surface of the lens holder are used as the driving coils which comprise these magnetic driving circuits. Also, the magnets for tracking opposite from the hollow coils for tracking are magnetized in the tracking direction, that is the direction of the circumference of the outer yoke. On the other hand, the magnets for focusing opposite from the hollow coils for focusing are magnetized in the focusing direction, that is the direction of the height of the outer yoke.
Additionally, in the objective lens driving apparatus of the contactingly movable axis type described above, the support shaft is formed of a metal and the like since the lens holder contactingly moves along the support shaft during driving; on the other hand, the whole lens holder is often formed of a resin material, such as polyphenylensulfide and the like, which has excellent contactingly movable characteristics. Also, a lens mounting portion is formed on the lens holder such that an objective lens can be horizontally mounted thereon. A reference surface for horizontal extension of the objective lens is formed at the lens mounting portion such that the objective lens is perpendicular to the support shaft. A flat flange portion of the objective lens contacts the reference surface such that the mounting position of the objective lens is determined. Therefore, the lens holder is designed such that the reference surface for horizontal extension of the objective lens is perpendicular to the axial line of the bearing hole.
Recently, along with demands for miniaturizing optical head apparatus, miniaturizing, and particularly, flattening of objective lens driving apparatus to be assembled therein is demanded.
In order to flatten an objective lens driving apparatus of the contacting movable axis type while maintaining a movable range of the objective lens in the focusing direction, the thickness of the lens holder needs to be thinner. In this case, it is necessary to replace the flat hollow coils for focusing and tracking adhered on the outer surface of the lens holder with smaller ones. However, application of smaller flat hollow coils lowers magnetic driving characteristics of the magnetic driving circuit. Hence, it has been difficult to flatten conventional objective lens driving apparatus of the contactingly movable axis type.
Moreover, in order to speed up pickup motion in the above objective lens driving apparatus by driving the lens holder at high speed, it is necessary to accomplish lightening and rigidifying the lens holder and to shift the higher resonant frequency of the lens holder to a higher range. However, a conventional lens holder is made of resin to increase contactingly movable performance of the bearing hole; thus, it is difficult to rigidify it. As a result, rigidity is increased by arranging the shape of the lens holder, such as thickening each part and forming a plurality of ribs. Nonetheless, complication of the lens holder shape makes difficult to flatten the lens holder; in addition, lowering of precision may result due to sinks of resin during resin formation. If precision in formation is lowered, essential performance of the objective lens driving apparatus is lowered. For example, the bearing hole cannot be accurately formed such that movement of the lens holder becomes rough; in addition, the axial line of the bearing hole cannot be accurately held at a perpendicular angle to the reference surface for horizontal extension of the objective lens such that the objective lens slants in relation to the support shaft. Therefore, it is not possible to accomplish both rigidifying the lens holder and increasing contactingly movable performance of the bearing hole in the conventional objective lens driving apparatus. If the shape of the lens holder becomes too complicated in order to improve these characteristics, it is problematic that miniaturizing and flattening of the lens holder are not accomplished and that the essential performance of the objective lens driving apparatus is impaired.